Method of forming an x-ray image using photostimulable phosphor

ABSTRACT

A method for forming an X-ray image. The method involves the use of an X-ray transforming sheet which includes a novel stimulable phosphor on a substrate. The phosphor has a high sensitivity to a semiconductor laser as a stimulating light and is represented by the formula: 
     
         {(M.sup.II X.sup.1.sub.2-2u X.sup.2.sub.2u).sub.1-x-y (M.sup.I 
    
      X 1   1-v  X 2   v ) x  (M III  X 1   3-3w  X 2   3w ) y  } 1-a  A a  :bEu 2+   
     where M II  represents at least one divalent metal such as Ba, Be, Mg or Ca; M 1  represents at least one monovalent metal such as Li, Na, K, Rb or Cs; M III  represents at least one trivalent metal such as Sc, La, Y, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Ga, In or Tl; X 1  represents Br or Cl; X 2  represents at least one halogen atom that is different from X 1  ; A represents at least one metal oxide such as BeO, MgO, CaO, SrO, BaO, ZnO, Al 2  O 3 , Y 2  O 3 , La 2  O 3 , In 2  O 3 , Ga 2  O 3 , SiO 2 , TiO 2 , ZrO 2 , GeO 2 , SnO 2 , Nb 2  O 5 , Ta 2  O 5  and ThO 2  ; and wherein 0&gt;x+y&gt;0.5; 0&gt;u+v+w&lt;0.1; 0&gt;a&gt;0.1; and 0&lt;b&gt;0.2. The transforming sheet is exposed to X-rays and then the exposed phosphor is stimulated with electromagnetic radiation to release the stored energy as a phosphostimulated luminescent light which may be detected to thereby obtain an image of the object.

This application is a continuation of application Ser. No. 08/098,722,filed Jul. 29, 1993, now abandoned, which is a divisional of applicationSer. No. 08/054,343, filed Apr. 7, 1993, now abandoned, which is acontinuation of application Ser. No. 07/585625, filed Sep. 20, 1990, nowabandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to stimulable phosphors, methods of makingsame, and X-ray image transforming sheets. More specifically, thepresent invention relates to stimulable phosphors activated by divalenteuropium, and the uses thereof. Such phosphors emit a light having apeak near to 400 nm when excited by radiation such as X-rays, electronbeams or ultraviolet rays, and further, release a light having a peaknear to 400 nm when later irradiated with visible or infrared light. Thelatter released light is called photostimulated luminescence, and isused for storing the energy of radiation and transforming it intovisible light. In particular, such released light is used fortransforming radiation images into visible images and one of the mostimportant applications thereof relates to X-ray image production formedical use.

2. Description of the Related Art

In X-ray image production using stimulable phosphors, X-rays transmittedthrough a part of a living body, such as a breast, are irradiated to astimulable phosphor sheet or panel containing a stimulable phosphor, andthe stimulable phosphor sheet or panel is then scanned by a laser beam.When excited by laser beams, stimulable phosphors release energy storedin crystals of the phosphor as stimulated luminescence ofnear-ultraviolet light corresponding to the stored energy. Thenear-ultraviolet light is detected by a detector such as a photoelectronmultificator and then transformed into an electric signal. The electricsignal is treated and displayed on a cathode ray tube (CRT), or is usedto modulate the intensity of another laser beam scanning and exposing asilver salt film, which is then developed to produce a visible image.

This method of transforming a laser beam into a visible image hasremarkable useful features, such as the following.

(1) It is possible to reduce the amount of X-ray exposure.

(2) A satisfactory image can be produced bypersons not skilled in theart, since stimulable phosphors have an X-ray sensitive range wider thanthat of a silver salt film, and thus the adjustment of the X-rayexposure for producing an image is easy.

(3) Since the image is transformable into an electric signal, variousimage treatments such as contour emphasizing can be easily effected.

(4) After producing an image, the X-ray image transforming sheet can beused repeatedly by irradiating the phosphor sheets with a stimulatinglight to release all stored energy and return the sheets to theiroriginal state.

Some stimulable phosphors used for these purposes, for example, ceriumand samarium-activated strontium sulfide phosphors (SrS:Ce,Sm), europiumand samarium-activated lanthanum oxisulfide phosphors (La₂ O₂ S:Eu,Sm),and manganese and halogen-activated zinc cadmium sulfide phosphors[(ZnCd)S:Mn,X; where X is a halogen], as disclosed in G.B. Patent No.1462769 or U.S. Pat. No. 3,859,527, are known. Japanese UnexaminedPatent Application (Kokai) No. 55-12143 discloses stimulable phosphorshaving the formula (Ba_(1-x-y) Mg_(x) Ca_(y))FX:Eu where X is Br or Cl;Japanese Unexamined Patent Publication (Kokai) No. 55-84389 disclosesstimulable phosphors having the formula BaFX:Ce,A where X is Cl, Br or Iand A is In, Tl, Gd, Sm or Zr; and Japanese Unexamined PatentPublication (Kokai) No. 60-84381 discloses stimulable phosphors havingthe formula MX₂ aMX'₂ :Eu where M is Ba, Sr or Ca and X and X' are Cl,Br or I.

Nevertheless, these stimulable phosphors are not adequate for intendedindustrial applications.

Namely, the stimulable phosphors disclosed in G.B. Patent No. 1,462,769and U.S. Pat. No. 3,859,527 have low X-ray sensitivity.

The stimulable phosphors disclosed in Japanese Unexamined PatentPublication (Kokai) Nos. 55-12143 and 55-84389 have practical readingsensitivities when subjected to stimulation by visible lasers, e.g.,helium neon lasers, but can not be stimulated by infrared semiconductorlasers made of a material such as GaAs, GaAlAs or InPGaAs, andtherefore, are not practical. The stimulable phosphors disclosed inJapanese Unexamined Patent Publication (Kokai) Nos. 60-84381 and55-12143 have spectrums of stimulated luminescence for reading thatextend to slightly longer wave lengths, but do not have sufficientcharacteristics for practical reading by semiconductor lasers.

The stimulable phosphors disclosed in Japanese Unexamined PatentPublication (Kokai) No. 60-84381 are MFXX':Eu. These phosphors aresimilar to the MFX:Eu phosphors disclosed in Japanese Unexamined PatentPublication (Kokai) No. 55-12143 except that a portion of the F isreplaced by a halogen X' which is different from F and X, but do nothave a practical sensitivity for reading by a semiconductor laser. Whenthe intensity of stimulated luminescence measured by stimulating thephosphors MFXX':Eu by light having a wave length of 780 nm is comparedwith that of MFX:Eu, it is shown that MFX:Eu exhibits a luminescenceintensity in a near-infrared wave length range that is larger than thealmost zero luminescence of the other stimulable phosphors. However, theluminescence intensity of MFX:Eu is still too low for practical use.

The object of the present invention is to provide novel stimulablephosphors capable of releasing luminescence having a practical intensitywhen stimulated by a near-infrared ray of a semiconductor laser. Thesemiconductor lasers of the present invention comprise those having ahigh output at a wave length longer than 500 nm, particularly longerthan 760-780 nm. Although semiconductor lasers having an output athigher wave lengths, i.e., visible light, have been developed,semiconductor lasers having a sufficient output for X-ray imagetransformation are limited to those having the above wave lengths, Morespecifically, an output power of at least 20 mW is necessary, whichlimits the semiconductor laser to the above lasers.

SUMMARY OF THE INVENTION

In accordance with the invention it has been found that noveleuropium-activated barium bromide and related phosphors have a practicalhigh infrared stimulated sensitivity allowing reading by stimulationwith semiconductor lasers. More specifically, the novel stimulablephosphors are represented by the following formula:

    {(M.sup.II X.sup.1.sub.2-2u X.sup.2.sub.2u).sub.1-x-y (M.sup.I X.sup.1.sub.1-v X.sup.2).sub.x (M.sup.III X.sup.1.sub.3-3w X.sup.2.sub.3w).sub.y }.sub.1-a A.sub.a :bEu.sup.2+

where M^(II) represents at least one divalent metal selected from thegroup consisting of Ba, Be, Mg and Ca; M^(I) represents at least onemonovalent metal selected from the group consisting of Li, Na, K, Rb andCs; M^(III) represents at least one trivalent metal selected from thegroup consisting of Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm,Yb, Lu, Al, Ga, In and Tl; X¹ represents an element selected from thegroup consisting of Br and Cl; X² represents at least one halogen atomthat is different from X¹ ; A represents at least one metal oxideselected from the group consisting of BeO, MgO, CaO, SrO, BaO, ZnO, Al₂O₃, Y₂ O₃, La₂ O₃, In₂ O₃, Ga₂ O₃, SiO₂, TiO₂, ZrO₂, GEO₂, SnO₂, Nb₂ O₅,Ta₂ O₅ and ThO₂ ; and wherein 0>x+y>0.5; 0>u+v+w<0.1; 0>a>0.1; and0<b>0.2.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an X-ray diffraction pattern of BaBr₂:Eu²⁺ ;

FIG. 2 is a diagram which illustrates the relationship between theintensity of the stimulated luminescence of BaBr₂ :Eu²⁺ and thewavelength of the stimulating light;

FIG. 3 is a diagram illustrating the relationship between the intensityof the stimulated luminescence of BaBr₂ :Eu²⁺ and the Eu content ofBaBr₂ :Eu²⁺ ;

FIG. 4 is a schematic sectional view of an X-ray transforming sheet;

FIG. 5 is a schematic illustration of a system for producing an X-rayimage;

FIG. 6 is a diagram illustrating the relationship between the intensityof the stimulated luminescence of BaCl₂ :Eu²⁺ and the wave length of thestimulating light;

FIG. 7 is a diagram illustrating the relationship between the intensityof the stimulated luminescence of Ba_(1-x) Ca_(x) Br₂ :Eu²⁺ and the wavelength of the stimulating light;

FIG. 8 is a diagram illustrating the relationship between the intensityof the stimulated luminescence of CaBr₂ :Eu²⁺ and the wave length of thestimulating light;

FIG. 9 is a diagram illustrating the relationship between the intensityof the stimulated luminescence of BaBr_(2-2u) Cl_(2u) :Eu²⁺ and theBaCl₂ content of BaBr_(2-2u) Cl_(2u) :Eu^(2+;)

FIG. 10 is a diagram illustrating the relationship between the intensityof the stimulated luminescence of (BaBr₂)(0.01V₂ GaO₃):Eu²⁺ and the wavelength of the stimulating light;

FIGS. 11 and 12 are schematic sectional views of X-ray transformingsheets;

FIG. 13 is a diagram which shows the percentage transmission of a glassplate with an anti-reflecting layer; and

FIG. 14 is a schematic sectional view of an X-ray transforming sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The stimulated luminescence phenomenon of the stimulable phosphors ofthe present invention is described with the use of BaBr₂ :Eu²⁺ as anexample. When irradiated with X-rays or ultraviolet rays, Eu, anemission center, emits a blue light when excited, i.e., it emitsphotoluminescence. Simultaneously, electrons in some Eu atoms areexcited and captured by color centers. The color center is a latticedefect of Br and is electrically positively charged, and thus theelectrons are captured by the color centers, which is the memory. Then,if the stimulable phosphor is irradiated with a red or infrared light,the electrons captured in the color centers are excited and returned tothe original Eu, while a blue light is emitted. This is known as astimulated luminescence. The intensity of the stimulated luminescencerelates to the intensity of the X-ray beam.

Europium is essential to the photostimulated luminescence of thestimulable phosphors of the present invention, but if the content ofeuropium is more than 20 mole %, the crystallinity of

    (M.sup.II X.sup.1.sub.2-2u X.sup.2.sub.2u).sub.1-x-y (M.sup.I X.sup.1.sub.1-v X.sup.2.sub.v).sub.x (M.sup.III X.sup.1.sub.3-3w X.sup.2.sub.3w).sub.y

is damaged and the intensity of photostimulated luminescence isundesirably lowered. Therefore, 0<b>0.2, preferably 0.0005>b>0.002, fromthe viewpoint of the intensity of the photostimulated luminescence.

The compound

    {(M.sup.II X.sup.1.sub.2-2u X.sup.2.sub.2u).sub.1-x-y (M.sup.I X.sup.1.sub.1-v X.sup.2.sub.v).sub.x (M.sup.III X.sup.1.sub.3-3w X.sup.2.sub.3w).sub.y }.sub.1-a A.sub.a :bEu.sup.2+

(wherein X¹ represents either Br or Cl and X² represents a halogen atomwhich is different from X¹) includes the following:

1) BaBr₂ :bEu²⁺.

2) BaCl₂ :bEu²⁺.

3) Ba_(1-u) M¹ _(u) Br₂ :bEu²⁺ where M¹ is at least one of Be, Mg and Caand 0<u>1. That is, Ba may be partly or completely replaced by Be, Mg,Ca or Sr, each of which is a divalent metal.

4) (BaBr₂)_(1-x) (M^(I) Br)_(x) :bEu²⁺ where M^(I) is at least one ofthe monovalent metals Li, Na, K, Rb and Cs, and 0<x>0.5, preferably0<x>0.05, more preferably 0<x>0.01. That is, Ba may be partly replacedby a monovalent metal.

5) (BaBr₂)_(1-y) (M^(III) Br₃)_(y) :bEu²⁺ where M^(III) is at least oneof the trivalent metals Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Al, Ga, In and Tl, and 0<y>0.5, preferably, 0<y>0.05, morepreferably 0<y>0.01. That is, Ba may be partly replaced by a trivalentmetal.

6) (BaBr₂)_(1-x-y) (M^(I) Br)(M^(III) Br₃):bEu²⁺ where M^(I) is at leastone of the monovalent metals Li, Na, K, Rb, and Cs, and M^(III) is atleast one of the trivalent metals Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy,Ho, Er, Tm, Yb, Lu, Al, Ga, In and Tl, and 0<x+y>0.5, preferably0<x+y>0.05, more preferably 0<x+y>0.01. That is, Ba may be partlyreplaced by a combination of a monovalent metal and a trivalent metal.

7) BaBr_(2-2u) X^(II) _(2u) :bEu²⁺ where X^(II) is at least one of F, Cland I and 0<u<0.1, preferably 0<u<0.08, more preferably 0<u<0.05. Thatis, Br may be partly replaced by a different halogen. It should be notedthat the phosphors of the present invention are different from thatknown BaXX' compounds (X and X' are different halogens), for example,BaBrCl. When a portion of one halogen, e.g., X¹, of BaXX¹ is replaced bythe other halogen X, the intensity of photostimulated luminescence isinitially remarkably reduced to near zero as the amount of thereplacement X is increased, but is raised again when the compositionbecomes BaX₂ (X is Br or Cl), or very close thereto. This clearlysuggests that BaX₂ (X is Br or Cl) is a photostimulated luminescencematerial that is essentially different from BaXX' (X and X' aredifferent halogens), i.e., not a modification or improvement of BaXX' byan addition of BaX₂ to BaXX' and thus, only a small amount of the secondhalogen X can be used in the present invention.

8) (BaBr₂)_(1-a) A_(a) :bEu²⁺ where A is at least one of the metaloxides BeO, MgO, CaO, SrO, BaO, ZnO, Al₂ O₃, Y₂ O₃, La₂ O₃, In₂ O₃, Ga₂O₃, SiO₂, TiO₂, ZrO₂, GEO₂, SnO₂, Nb₂ O₅, Ta₂ O₅ and ThO₂, and 0<a>0.1,preferably 0<a>0.05, more preferably 0<a>0.01. That is, a small amountof a metal oxide may be included.

9) Any combination of compounds 3) to 8). For example, (BaBr_(2-2u) X²_(2u))_(1-a) A_(a) :bEu²⁺.

10) Compounds similar to compounds 3) to 9), wherein Br₂ is replaced byCl₂. That is, BaCl₂ may be used as the base compounds instead of BaBr₂.

The above stimulable phosphors may be prepared by firing a mixture ofM^(II) X¹ ₂ and an Eu source, e.g., EuX₂ (X is a halogen, particularlyBr or Cl) Optionally, M^(II) X² ₂, M^(I) X¹, M^(I) X², M^(III) X¹ ₃,M^(III) X² ₃ and/or A may be included in the mixture. The startingmaterials may be hydrates of the specified compounds.

The mixing can be carried out by dry mixing in a ball mill, by a wetprocedure wherein the starting materials are dissolved in water and thendried under vacuum or in air, or heated to remove water and obtain a drymixture, or other procedures. When a wet process is used, the obtainedmixture is dried in air at 200°-600° C. or in vacuum at 100°-300° C.,and the mixture is then preferably fired in a reducing atmospherecontaining hydrogen. Alternatively, the original firing atmosphere maybe an inactive atmosphere or an oxidizing atmosphere to ash and removethe binder, and such firing should then be followed by reducing the Euin a reducing atmosphere. A suitable reducing atmosphere is an inert gasor nitrogen containing not more than 30.001% hydrogen, for example, theatmosphere may include nitrogen gas, argon gas or helium gas, andpreferably is a mixture of helium and hydrogen. The firing temperaturedepends on the types and compositions of the starting materials, but500°-1000° C. is generally adequate, with 700°-900° C. being preferable,as in conventional procedures. The firing time depends on the types andcompositions of the starting materials, the amount of the startingmaterials charged into the refractory container, and the firingtemperature, etc., but is generally 30 minutes to 48 hours, andpreferably is 1-12 hours, at the above firing temperature. After firing,the phosphor is sintered and then must be subjected to pulverization andscreening. The pulverization and screening are preferably carried out ina dry inactive gas atmosphere to prevent absorption of moisture; andthis is also preferable during the mixing of the starting materials.

The stimulable phosphor is coated onto a support to form an X-raytransforming sheet.

The support may be a Mylar film, a plastic plate, or a ceramic plate,etc., and may include a carbon black layer for improving the resolvingpower, a reflecting layer of aluminum for improving the sensitivity, anda protecting layer for preventing moisture absorption, etc., on thesurface thereof.

The formation of a stimulable phosphor layer on a support can beperformed by coating a paste of a stimulable phosphor, a binder and asolvent onto the support, followed by drying and curing the binder, ifnecessary, by heating irradiation. The binder may be, for example, acrylresin, epoxy resin, urethane resin, phenol resin, nylon resin, teflonresin, or polyester resin, etc. The phosphor preferably has a particlesize of less than 100 μm, more preferably less than 30 μm. Thestimulable phosphor is preferably mixed with a binder in an amount of10-99% by weight, more preferably 80-95% by weight, of the stimulablephosphor.

The stimulable phosphor layer may be a layer of a single stimulablephosphor or a mixture of stimulable phosphors including conventionalstimulable phosphors, or a multiple stimulable phosphor layer with otherstimulable phosphor layers.

If necessary, the coating of the stimulable phosphor is performed byadjusting the viscosity of the paste with a solvent, followed by coatingthe paste on the support using a doctor blade or a roll coaster, etc.The thickness of the coating (dry) is 50-500 μm, preferably 150-300 μm.

The surface of the stimulable phosphor layer is preferably covered by aprotecting layer, etc., and as the protecting layer, a glass layer, aMylar film, or a polyethylene terephthalate film having an indium tinoxide (ITO) layer, etc., may be used.

The method of producing an X-ray image of the present invention is thesame as the conventional method, except that the stimulable phosphorused is novel and a semiconductor laser may preferably be used as thesource of the stimulating light. Namely, an X-ray image transformingsheet is irradiated with X-ray image transforming sheet is irradiatedwith X-rays passed through an object such as a human body, to causeabsorption of the transmitted X-rays by the X-ray image transformingsheet. Then, the X-ray image transforming sheet is irradiated withelectromagnetic radiation having a wave length of 500-1000 nm, torelease the radiation energy stored in the X-ray image transformingsheet as photostimulated luminescence. This photostimulated luminescenceis in the form of an image corresponding to the X-ray image transmittedthrough the object, and therefore, by visualization of thephotostimulated luminescence, an X-ray image can be obtained. Thevisualization process may be performed by conventional photography, butis preferably made by scanning a stimulating beam on the X-ray imagetransforming sheet, detecting the released stimulated luminescence,transforming the intensity of the detected luminescence into an electricsignal, and visualizing the electric signal by electric means. In thislatter method, various image treatments can be effected on an X-rayimage.

The X-ray image transforming sheet of the present invention ischaracterized by the capability of using a semiconductor laser as astimulating light source. Semiconductor lasers having a wave length of670-680 nm, 780 nm, 830 nm or 900 nm have been developed and put to apractical use, and preferably these semiconductor lasers are used. Theknown stimulable phosphors are sensitive mainly to stimulating lighthaving shorter wave lengths, and therefore, cannot produce a desiredluminescence having a sufficient intensity by a semiconductor laserhaving a limited output power, and thus only a helium-neon laser havinga wave length of 630 nm or a laser having a shorter wave length can beused in a practical sense at present. In contrast, when the stimulablephosphors of the present invention are used, the obtainedphotostimulated luminescence has an intensity which is more than 1.5times (depending on the wave length, more than several tens of times) asgreat as that obtainable using conventional stimulable phosphors.Accordingly, X-ray images can be obtained using X-ray exposures within arange which is tolerable to the human body and a semiconductor laser.Further, the X-ray exposure needed for obtaining X-ray images usingsemiconductor lasers can be reduced in comparison with conventionalstimulable phosphors.

EXAMPLES Example 1

0.998 mole of 99,999% and 95% purity BaBr₂ powders were each mixedseparately with 0.001 mole of EuBr₃ powder in a ball mill for 6 hours,the mixture was dried in the ball mill by opening a cap thereof, pullinga vacuum and heating at 100° C. for 2 hours, and the mixture was furthermilled for 6 hours.

The mixed phosphor starting materials were charged into a quartz boat,mounted in a quartz tube, and fired at 850° C. for 2 hours. The firingatmosphere was provided by a gas flow of hydrogen at 10 cc/min andhelium at 10 l/min. After firing, the center of the quartz tube wascooled to 400° C., the boat was moved from a high temperature portion toa low temperature portion in the tube, and was allowed to cool to roomtemperature in the tube. The obtained phosphor was lightly crushed in anagate mortar.

FIG. 1 shows the X-ray diffraction pattern of the obtained phosphor.This phosphor has a crystal structure of BaBr₂. The bright line spectrumanalyzed on the phosphor had a blue emission peak at 400 nm, which wasconfirmed to be an emission from Eu²⁺.

For comparison, a known BaClBr:0.001Eu²⁺ compound was prepared in asimilar manner.

These phosphors were charged in a measurement cell having a quartz glasswindow and the spectrum of the stimulated luminescence was analyzed. Thelight used for stimulation was a spectral light obtained by passing alight from a halogen lamp through a spectrograph. The phosphor in thequartz cell was irradiated with the stimulating light, and the lightreleased from the phosphor as photostimulated luminescence wasintroduced to a photomultiplier tube through a filter which preventstransmission of the stimulating light therethrough. The introduced lightwas photoelectrically transferred by the photomultiplier tube todetermine the intensity of the stimulated luminescence. The phosphorpacked in the cell was irradiated with an X-ray beam emitted from anX-ray tube (a tube voltage of 80 kV and a d-current of 200 mA) located 1m away from the phosphor (the irradiation time was 0.5 second). Theirradiated phosphor was then placed in the above-mentioned measurementcell and analyzed, and the thus-obtained spectrum of the photostimulatedluminescence is shown in FIG. 2.

FIG. 2 demonstrates that, when stimulated by a light having a wavelength of 780 nm, BaBr₂ :Eu²⁺ exhibits a photostimulated luminescencehaving an intensity about 5 times higher than that of BaClBr:Eu²⁺. Thesensitivity of the phosphor was lowered by only about 10% when the 95%purity phosphor was compared with the 99.999% purity phosphor.

Example 2

A phosphor was prepared in the same manner as in Example 1, except thatthe ratio of EuBr₃ was varied, and the intensity of the photostimulatedluminescence thereof was measured. FIG. 3 shows the intensity of thephotostimulated luminescence of the phosphor when stimulated by asemiconductor laser having a wave length of 780 nm.

Example 3

To 500 g of the BaBr₂ :E²⁺ of Example 1 were added 30 g ofpolymethylmethacrylate, 3 g of dibutylphthalate, and 150 g of toluene,and the mixture was mixed in a ball mill for 20 hours.

Referring to FIG. 4, the thus-obtained paste of the phosphor was coatedon a support 1 of a Mylar film by a doctor blade, and dried to form aphosphor layer 2 having a thickness of 200 μm. The top and side surfacesof the phosphor layer 2 were covered by an ITO layer and an epoxy resinadhesive, to form a protecting layer 3.

The obtained X-ray image transforming sheet was irradiated with a 10 mRX-ray dose followed by scanning with a laser beam from a 10 mWsemiconductor laser (wave length 780 nm), to measure the photostimulatedluminescence. For comparison, an X-ray image transforming sheet usingBaClBr:Eu²⁺ as the phosphor was made and measured in the same manner asin Example 3.

The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                       Intensity of photostimulated                                   Material       luminescence                                                   ______________________________________                                        BaClBr:0.001Eu.sup.2+                                                                        100                                                            BaBr.sub.2 :0.001Eu.sup.2+                                                                   480                                                            ______________________________________                                    

Example 4

The X-ray image transforming sheet of Example 3 was installed in therecording and reading apparatus shown in FIG. 5, and a transmitted imageof a human breast was produced. In FIG. 5, 4 denotes an X-ray source, 5an X-ray image transforming sheet, 6 a laser source, 7 a collector, 8 afilter, 9 a photoelectric transfer device, 10 an image reproducingdevice, 11 an image display device, and 12 a human body (an object). Aclear image was produced at a voltage of 100 V, an irradiation distanceof 2 m, an X-ray irradiation amount of 10 MAS, and a reading laser lightof 780 nm and 10 mW.

Example 5

First, 208.2 g of BaCl₂ and 0.26 g of EuCl₃ were mixed in a ball millfor 6 hours, dried in vacuum at 100° C. for 1 hour, and again ballmilled for 6 hours. The resultant mixture was charged into a quartz boatand fired at 880° C. in a reducing atmosphere for 2 hours. Theatmosphere was a helium gas containing hydrogen.

The intensity of the stimulated luminescence of the obtained phosphorBaCl₂ :0.001Eu²⁺ was measured by irradiation with a 10 mR X-ray dosefollowed by scanning with a spectroscopic light (wave length 500-900nm). The result is shown in FIG. 6. The peak is near 670 nm.

Examples 6 and 7

The starting material having a composition shown in Table 2 were mixedin a ball mill for 6 hours, dried in vacuum at 100° C. for 1 hour, andagain ball milled for 6 hours. The mixture was charged into a quartzboat and fired at 830° C. in a reducing atmosphere for 2 hours. Theatmosphere was a helium gas containing hydrogen. For comparison, thesame stimulable phosphor without additional CaBr₂ or MgBr₂ was prepared.

                  TABLE 2                                                         ______________________________________                                                Starting  Weight                                                      Example Material  (g)      Composition                                        ______________________________________                                        6       BaBr.sub.2                                                                              881      (Ba.sub.0.8 Ca.sub.0.2)Br.sub.2 :0.001Eu.sup.2+                               3                                                          CaBr.sub.2                                                                              119                                                                 EuBr.sub.3                                                                              2.23                                                        7       BaBr.sub.2                                                                              856      (Ba.sub.0.8 Mg.sub.0.2)Br.sub.2 :0.001Eu.sup.2+                               4                                                          MgBr.sub.2                                                                              144                                                                 EuBr.sub.3                                                                              2.23                                                        ______________________________________                                    

The intensity of the stimulated luminescence of the obtained phosphorswas measured by irradiation with a 10 mR X-ray dose followed by scanningwith a spectroscopic light (wave length 500-900 nm). The results areshown in FIG. 7. When the amount of added CaBr₂ was varied, thesensitivity of the luminescence stimulated by a 780-830 nm light wasimproved 1.2-1.5 times.

Example 8

First, 199.7 g of CaBr₂ and 0.39 g of EuBr₃ were mixed in a ball millfor 6 hours, dried in vacuum at 100° C. for 1 hour, and again ballmilled for 6 hours. The mixture was charged into a quartz boat and firedat 740° C. in a reducing atmosphere for 2 hours. The atmosphere was ahelium gas containing hydrogen.

The intensity of the stimulated luminescence of the obtained phosphorCaBr₂ :0.001Eu²⁺ was measured by irradiation with a 10 mR X-ray dosefollowed by scanning with a spectroscopic light (wave length 500-900nm). The result is shown in FIG. 8. The peak is near 540 nm.

Example 9

Predetermined amounts of BaBr₂, EuBr₃ and BaCl₂ were mixed in a ballmill for 6 hours, dried in vacuum at 100° C. for 1 hour, and again ballmilled for 6 hours. The amount of BaCl₂ was varied. The mixture wascharged into a quartz boat and fired at 840° C. in a reducing atmospherefor 2 hours. The atmosphere was a helium gas containing hydrogen.

The intensity of the stimulated luminescence of the obtained phosphorBaBr_(2-2u) Cl_(2u) :0.001Eu²⁺ (0>u>0.1) was measured by irradiationwith a 10 mR X-ray dose followed by scanning with a 100 mW argon ionlaser (wave length 532 nm). The results are shown in FIG. 9. It is seenthat the intensity of the stimulated luminescence can be improved toabout 15 times that of BaBrCl:0.001Eu²⁺ and further, as seen in FIG. 3,the intensity can be improved to about 1.5 times by controlling thecontent of Eu.

Example 10

Example 9 was repeated except that the firing atmosphere was a nitrogenatmosphere containing hydrogen.

Example 11

Example 9 was repeated except that the starting materials were first wetmixed and then dried.

Example 12

Example 9 was repeated except that the starting materials were firstdissolved in water, filtered, wet mixed, and then dried. The intensitiesof the stimulated luminescence of the phosphors obtained in Examples9-12 were each measured by irradiation with a 10mR X-ray dose followedby scanning with a 10 mW semiconductor laser (wave length 780 nm). Theresults (relative intensities when the intensity of Example 9 is set at100) are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Example No.    Intensity                                                      ______________________________________                                        9              100                                                            10             80                                                             11             135                                                            12             145                                                            ______________________________________                                    

Examples 13-16

Predetermined amounts of BaBr₂, EuBr₃ and NaBr were mixed in a ball millfor 6 hours, dried in a vacuum at 100° C. for 1 hour, and then againball milled for 6 hours. The amount of NaBr was varied. The mixture wascharged into a quartz boat and fired in a reducing atmosphere at 840° C.for 2 hours. The atmosphere was a helium gas containing hydrogen. Theintensities of the stimulated luminescence of the obtained phosphorsBa_(1-x) Na_(x) Br_(2-x) :0.001Eu²⁺ (0>x>0.04) were measured byirradiation with a 10 mR X-ray dose followed by scanning with a 10 mWsemiconductor laser (wave length 780 nm). The results are shown in Table4. The intensities are relative values when the intensity of BaBr₂:0.001Eu²⁺ is set at 100.

                  TABLE 4                                                         ______________________________________                                        Example No. Composition       Intensity                                       ______________________________________                                        13          BaBr.sub.2 :0.001Eu.sup.2+                                                                      100                                             14          Ba.sub.0.99 Na.sub.0.01 Br.sub.1.99 :0.001Eu.sup.2+                                             108                                             15          Ba.sub.0.98 Na.sub.0.02 Br.sub.1.98 :0.001Eu.sup.2+                                             135                                             16          Ba.sub.0.96 Na.sub.0.04 Br.sub.1.96 :0.001Eu.sup.2+                                             117                                             ______________________________________                                    

Example 17

First, 294.2 g of BaBr₂, 0.39 g of EuBr₃, and 0.58 g of NaCl were mixedin a ball mill for 6 hours, dried in vacuum at 100° C. for 1 hour, andagain ball milled for 6 hours. The mixture was charged into a quartzboat and fired in a reducing atmosphere at 840° C. for 2 hours. Theatmosphere was a helium gas containing hydrogen.

The intensity of the stimulated luminescence of the obtained phosphorBa₀.99 Na₀.01 Br₁.98 Cl₀.01 :0.001Eu²⁺ was measured by irradiation witha 10 mR X-ray dose followed by scanning with a 10 mW semiconductor laser(wave length 780 nm). The resulting intensity was 120, as a relativevalue, when the intensity of BaBr₂ :0.001Eu²⁺ was set at 100.

Examples 18-22

Similar results were obtained when Na was replaced by Li or K inExamples 13-17.

Examples 23-25

Predetermined amounts of BaBr₂, EuBr₃ and GdBr₃ were mixed in a ballmill for 6 hours, dried in vacuum at 100° C. for 1 hour, and again ballmilled for 6 hours. The mixture was charged into a quartz boat and firedin a reducing atmosphere at 840° C. for 2 hours. The atmosphere was ahelium gas containing hydrogen.

The intensities of the stimulated luminescence of the obtained phosphorsBa_(1-y) Gd_(y) Br_(2+y) :0.001Eu²⁺ (0>y>0.04) were measured byirradiation with a 10 mR X-ray dose followed by scanning with a 10 mWsemiconductor laser (wave length 780 nm). The results are shown in Table5. The intensities are relative values when the intensity of BaBr₂:0.001Eu²⁺ is set at 100.

                  TABLE 5                                                         ______________________________________                                        Example No. Composition       Intensity                                       ______________________________________                                        13          BaBr.sub.2 :0.001Eu.sup.2+                                                                      100                                             23          Ba.sub.0.99 Gd.sub.0.01 Br.sub.2.01 :0.001Eu.sup.2+                                             120                                             24          Ba.sub.0.98 Gd.sub.0.02 Br.sub.2.02 :0.001Eu.sup.2+                                             132                                             25          Ba.sub.0.96 Gd.sub.0.04 Br.sub.2.04 :0.001Eu.sup.2+                                             118                                             ______________________________________                                    

Example 26

First, 294.2 g of BaBr₂, 0.39 g of EuBr₃ and 2.64 g of GdCl₃ were mixedin a ball mill for 6 hours, dried in vacuum at 100° C. for 1 hour, andagain ball milled for 6 hours. The mixture was charged into a quartzboat and fired in a reducing atmosphere at 840° C. for 2 hours. Theatmosphere was a helium gas containing hydrogen.

The intensity of the stimulated luminescence of the obtained phosphorBa₀.99 Gd₀.01 Br₁.98 Cl₀.03 :0.001Eu²⁺ was measured by irradiation witha 10 mR X-ray dose followed by scanning with a 10 mW semiconductor laser(wave length 780 nm). The resulting intensity was 120, as a relativevalue, when the intensity of BaBr₂ :0.001Eu²⁺ was set at 100.

Examples 27-30

Similar results were obtained when Gd was replaced by Y or Ga inExamples 23-25.

Examples 31-33

Predetermined amounts of BaBr₂, EuBr₃, NaBr and GdBr₃ were mixed in aball mill for 6 hours, dried in vacuum at 100° C. for 1 hour and againball milled for 6 hours. The amount of GdBr₃ was varied. The mixture wascharged into a quartz boat and fired in a reducing atmosphere at 840° C.for 2 hours. The atmosphere was a helium gas containing hydrogen.

The intensities of the stimulated luminescence of the obtained phosphorsBa_(1-x-y) Na_(x) Gd_(y) Br_(2-x+y) :0.001Eu²⁺ (0>x, y>0.02) weremeasured by irradiation with a 10mR X-ray dose followed by scanning witha 10 mW semiconductor laser (wave length 780 nm). The results are shownin Table 6. The intensities shown in Table 6. The intensities shown inTable 6 are relative values when the intensity of BaBr₂ :0.001Eu²⁺ isset at 100.

                  TABLE 6                                                         ______________________________________                                        Example No.                                                                             Composition          Intensity                                      ______________________________________                                        13        BaBr.sub.2 :0.001Eu.sup.2+                                                                         100                                            31        Ba.sub.0.99 Na.sub.0.005 Gd.sub.0.005 Br.sub.2 :0.001Eu.sup.2+                                     128                                            32        Ba.sub.0.98 Na.sub.0.01 Gd.sub.0.01 Br.sub.2 :0.001Eu.sup.2+                                       132                                            33        Ba.sub.0.96 Na.sub.0.02 Gd.sub.0.02 Br.sub.2 :0.001Eu.sup.2+                                       118                                            ______________________________________                                    

Example 34

First, 294.2 g of BaBr₂, 0.39 g of EuBr₃, 1.32 g of GdCl₃ and 0.29 g ofNaCl were mixed in a ball mill for 6 hours, dried in vacuum at 100° C.for 1 hour, and again ball milled for 6 hours. The mixture was chargedinto a quartz boat and fired in a reducing atmosphere at 840° C. for 2hours. The atmosphere was a helium gas containing hydrogen. Theintensity of the stimulated luminescence of the obtained phosphor Ba₀.99Na₀.005 Gd₀.005 Br₁.98 Cl₀.02 :0.001Eu²⁺ was measured by irradiationwith a 10 mR X-ray dose followed by scanning with a 10 mW semiconductorlaser (wave length 780 nm). The resulting intensity was 115, as arelative value, when the intensity of BaBr₂ :0.001Eu²⁺ was set at 100.

Examples 35-37

Similar results were obtained when Gd was replaced by Y or Ga and/or Nawas replace by Li or K in Examples 31-33.

Examples 38-54

Predetermined amounts of BaBr₂, EuBr₃ and each metal oxide A shown inTable 7 were mixed in a ball mill for 6 hours, dried in vacuum at 100°C. for 1 hour, and again ball milled for 6 hours. The ratio of the metaloxide A was 1.0 mole % and that of the Eu element was 0.1 mole %. Themixture was charged into a quartz boat and fired in a reducingatmosphere at 840° C. for 6 hours. The atmosphere was a helium gas (5l/min) containing hydrogen (5 cc/min).

The intensity of the stimulated luminescence of the obtained phosphor(BaBr₂)(0.01Ga₂ O₃):0.001Eu²⁺ was measured by irradiation with a 10 mRX-ray dose followed by scanning with various wave lengths. The resultsare shown in FIG. 10.

The intensities of the stimulated luminescence of the obtained phosphors(BaBr₂)0.01A:0.001Eu²⁺ were measured by irradiation with a 10 mR X-raydose followed by scanning with a 10 mW semiconductor laser (wave length780 nm). The results are shown in Table 7. The intensities shown arerelative values when the intensity of BaBr₂ :0.001Eu²⁺ is set at 100.

                  TABLE 7                                                         ______________________________________                                        Example   Metal        Peak     Intensity                                     No.       Oxide A      Intensity                                                                              at 780 nm                                     ______________________________________                                        13        --           100      52                                            38        Ga.sub.2 O.sub.3                                                                           105      61                                            39        Al.sub.2 O.sub.3                                                                           103      59                                            40        SiO.sub.2    101      59                                            41        MgO          98       58                                            42        CaO          95       56                                            43        BaO          101      60                                            44        ZnO          95       55                                            45        Y.sub.2 O.sub.3                                                                            103      60                                            46        La.sub.2 O.sub.3                                                                           97       56                                            47        In.sub.2 O.sub.3                                                                           95       55                                            48        TiO.sub.2    102      59                                            49        ZrO.sub.2    105      60                                            50        GeO.sub.2    103      60                                            51        SnO.sub.2    93       53                                            52        Nb.sub.2 O.sub.5                                                                           95       54                                            53        Ta.sub.2 O.sub.5                                                                           92       51                                            54        ThO.sub.2    93       52                                            ______________________________________                                    

Example 55

X-ray image transforming sheets having a structure shown in FIG. 11 weremade. In FIG. 11, the reference numeral 11 denotes a support, 12 astimulable phosphor layer, 13 a protecting layer, and 14 an adhesive.

A moisture permeation test was performed on the materials used for theprotecting layer 13 and support 11 of the above X-ray image transformingsheets, under JIS/Z0208. The temperature was 40° C. and the humidity was90%. The sheet materials used were a glass plate having a thickness of80 μm, an aluminum plate having a thickness of 0.2 mm, and a stainlesssteel plate having a thickness of 0.2 mm. For comparison, a polyethyleneterephthalate (PET) film having a thickness of 50 μm and a Saran UB filmhaving a thickness of 25 μm were used.

The results are shown in Table 8.

                  TABLE 8                                                         ______________________________________                                                       Humidity Permeation                                            Sample Measured                                                                              (g/m.sup.2 24h)                                                ______________________________________                                        PET film       8                                                              Saran UB       0.7                                                            Glass plate    less than 0.001                                                Aluminum plate less than 0.001                                                Stainless plate                                                                              less than 0.001                                                ______________________________________                                    

It is seen that inorganic plates made of metal or glass provideexcellent protecting layers.

Example 56

X-ray image transforming sheets having a structure shown in FIG. 12 weremade. On a glass plate 21 having a thickness of 1.1 mm, a dielectricreflecting layer 22 of SiO₂, AlN, SnO₂, In₂ O₃, ZnO, Si₃ N₄, TiO₂, MgF₂or LiF having a thickness of about 50 nm was formed by evaporation orsputtering and then a stimulable phosphor layer 23 of BaBr₂ :Eu²⁺ havinga thickness of about 200 μm and a glass protecting layer 24 having athickness of 700 μm were formed.

The intensities of the stimulated luminescence of the obtained X-rayimage transforming sheets were measured by irradiation with a 10 mRX-ray dose followed by scanning with a 10 mW semiconductor laser (wavelength 780 nm). The results are shown in Table 9. The intensities shownare relative values when the intensity of the sheet without a dielectricreflecting layer is set at 100.

                  TABLE 9                                                         ______________________________________                                                       Intensity of stimulated                                        Sample Measured                                                                              luminescence                                                   ______________________________________                                        SiO.sub.2      1.30                                                           AlN            1.35                                                           SnO.sub.2      1.50                                                           In.sub.2 O.sub.3                                                                             1.45                                                           ZnO            1.45                                                           ______________________________________                                    

Similar results were obtained when the supports had thicknesses of 1.5mm.

Example 57

The light transmittances of glass plates having a thickness of 0.5 mmand coated on both surfaces with an anti-reflecting multilayer (20layers of MgF₂ having a thickness of 100-300 nm) or an anti-reflectingsingle layer (a layer of MgF₂ having a thickness of 100 nm and adjustedto a wave length of 780 nm) were measured and shown as the curves A andB, respectively, in FIG. 13. For comparison, the light transmittance ofthe same glass plate without an anti-reflecting layer was measured andshown as the curve C in FIG. 13. It is seen in FIG. 13 that thetransmittance was improved by about 7-9% when the anti-reflectingmultilayer was applied, as compared to the case without ananti-reflecting layer. The transmittance was improved by about 2-3% whenthe anti-reflecting single layer was used, as compared to the case wherethe anti-reflecting multilayer was used.

X-ray image transforming sheets having a structure shown in FIG. 14 weremade. Support 31 was a glass plate having a thickness of 1.11 mm, and astimulable phosphor layer 32 of BaBr₂ :Eu²⁺ (0.3 mm thick) was coatedthereon as a paste containing an acryl resin using a doctor blade.Further, a glass plate (0.5 mm thick) 33 was coated with ananti-reflecting layer 34 as described above and was mounted on thestimulable phosphor layer 2 using an adhesive 35. For comparison, aglass plate without an anti-reflecting layer was used on another sheet.

The sensitivities of these X-ray image transforming sheets are shown inTable 10. The measurements were performed by irradiation with a 10 mRX-ray dose followed by scanning with a 10 mW semiconductor laser (wavelength 780 nm).

                  TABLE 10                                                        ______________________________________                                        Protecting layer     Sensitivity                                              ______________________________________                                        Glass only           1000                                                     Glass with an anti-reflecting                                                                      1100                                                     multilayer (one side)                                                         Glass with a single anti-reflecting                                                                1170                                                     layer (one side)                                                              Glass with a single anti-reflecting                                                                 1180.                                                   layer (both sides)                                                            ______________________________________                                    

We claim:
 1. A method of forming an X-ray image comprising:providing anX-ray image transforming sheet which comprises a stimulable phosphor ona substrate, said stimulable phosphor exhibiting photostimulatedluminescence when excited by visible or infrared light, said phosphorbeing represented by the formula:

    BaBr.sub.2 :bEu.sup.2+,

wherein 0<b>0.2; exposing said transforming sheet to X-rays passedthrough an object to thereby cause said X-rays to be absorbed by saidphosphor; stimulating the exposed transforming sheet withelectromagnetic waves having a wave length of 500 to 1000 nm to releasethe energy stored in the phosphor as a photostimulated luminescentlight; and detecting the released photostimulated luminescent light toobtain an image of the object.
 2. A method of forming an x-ray image asset forth in claim 1, wherein the electromagnetic waves for stimulatingthe exposed transforming sheet have a wavelength which is either 780 nm,830 nm or 900 nm, or which is in the range of 670 to 680 nm.
 3. A methodof forming an X-ray image comprising:providing an X-ray imagetransforming sheet which comprises a stimulable phosphor on a substrate,said stimulable phosphor exhibiting photostimulated luminescence whenexcited by visible or infrared light, said phosphor being represented bythe formula:

    (BaBr.sub.2).sub.1-a.A.sub.a : bEu.sup.2+,

where A represents at least one metal oxide selected from the groupconsisting of BeO, MgO, CaO, SrO, BaO, ZnO, Al₂ O₃, Y₂ O₃, La₂ O₃, In₂O₃, Ga₂ O₃, SiO₂, TiO₂, ZrO₂, GeO₂, SnO₂, Nb₂ O₅, Ta₂ O₅ and ThO₂ ; andwherein 0<a>0.1 and 0<b>0.2; exposing said transforming sheet to X-rayspassed through an object to thereby cause said X-rays to be absorbed bysaid phosphor; stimulating the exposed transforming sheet withelectromagnetic waves having a wave length of 500 to 1000 nm to releasethe energy stored in the phosphor as a photostimulated luminescentlight; and detecting the released photostimulated luminescent light toobtain an image of the object.
 4. A method of forming an x-ray image asset forth in claim 3, wherein the electromagnetic waves for stimulatingthe exposed transforming sheet have a wavelength which is either 780 nm,830 nm or 900 nm, or which is in the range of 670 to 680 nm.